Strips a leading File: or Image: from a file name, and leaves anything else alone. Used by Template:Video so that {{video|File:clip.mp4}} works as well as {{video|clip.mp4}}.

It is here because the failure it prevents is a silent one. {{filepath:}} wants the bare name and returns nothing at all for a prefixed one, so the player comes out with an empty src: a video box of the right size, with controls that do nothing, and not a word anywhere about why. Writing File: in front of a file name is the natural thing to do on a wiki — every other template that takes one is written [[File:x.png]] — so it should not be the thing that breaks it.